Help! 1mo FF DS suddenly only taking half the amount of formula and then falling asleep

Until now DS (1mo, a big baby, currently 11lb) has been having 5-6oz of formula at each feed, 6 times per day.

Since yesterday he has started falling asleep after 3oz and his feeding patterns have gone all over the shop. He still screams for his food like he's really hungry but then just seems to lose interest.

He seems to be doing this 8 or so times in 24 hours, and is still having about 1 big feed per day, so he is still getting enough food over 24 hours I reckon (he had about 29oz yesterday), but I wonder if there's something wrong with him. Is he poorly? Has he gone off his milk? Or is this just a phase? Anyone else known a FF baby do this?